1. What is the working principle of a displacer level transmitter?
A displacer level transmitter operates based on Archimedes’ principle. It uses a displacer element, a weighted cylinder or sphere, that is suspended in the process liquid. As the liquid level rises, the displacer experiences a buoyant force that is proportional to the amount of liquid it displaces. This buoyant force reduces the apparent weight of the displacer. The transmitter detects this change in weight and converts it into a proportional output signal, typically 4-20 mA, which corresponds to the liquid level.
2. What is the purpose of a displacer in a level transmitter?
The displacer is the primary sensing element in a displacer level transmitter. Its purpose is to experience a change in buoyant force as the liquid level changes. This change in buoyancy, which directly correlates to the liquid level, is what the transmitter measures to determine the level.
3. What is a stilling well, and when is it used with a displacer level transmitter?
A stilling well is a vertically mounted pipe with open ends that is placed inside a tank or vessel. It houses the displacer and protects it from turbulence, agitation, or foaming within the process liquid. This ensures a more stable and accurate level measurement by providing a relatively calm liquid surface around the displacer.
4. How do you calibrate a displacer level transmitter?
Calibration involves setting the zero and span adjustments of the transmitter to correspond to the 0% and 100% level points. The general steps are:
- Zero Adjustment (0% Level): With the displacer hanging freely in the air (or with the liquid at its lowest point), the transmitter’s output is adjusted to the lower range value (e.g., 4 mA).
- Span Adjustment (100% Level): With the displacer fully submerged in the process liquid (or with the liquid at its highest point), the transmitter’s output is adjusted to the upper range value (e.g., 20 mA). This can also be simulated by hanging a known weight from the displacer.
5. How do you simulate a 100% level condition for calibration when the displacer is hanging freely in the air?
To simulate a 100% level condition without the actual process liquid, you can hang a weight from the displacer. The weight should be equal to the buoyant force that would be exerted on the displacer when it is fully submerged in the process liquid. This weight can be calculated using the formula: Weight = Volume of Displacer x Density of Liquid.
6. How do you calculate a 5-point calibration table for a displacer level transmitter?
A 5-point calibration checks the transmitter’s linearity at 0%, 25%, 50%, 75%, and 100% of its measurement range. To create a calibration table, you need to determine the expected output signal at each of these points. For a 4-20 mA output, the corresponding values would be:
- 0%: 4 mA
- 25%: 8 mA
- 50%: 12 mA
- 75%: 16 mA
- 100%: 20 mA
You would then apply the corresponding level (or simulated weight) for each point and verify that the transmitter’s output is within the acceptable tolerance.
7. What is a torque tube in a displacer level transmitter, and what are its functions?
A torque tube is a mechanical component that connects the displacer to the transmitter’s sensing mechanism. Its primary functions are:
- It supports the weight of the displacer.
- It converts the vertical movement of the displacer into a small rotational movement as the buoyant force changes.
- It provides a seal between the process and the transmitter electronics.
8. What factors need to be considered when selecting a torque tube material?
The selection of a torque tube material is critical for reliable operation. Key factors to consider include:
- Corrosion resistance: The material must be compatible with the process fluid to prevent corrosion.
- Process temperature and pressure: The material must be able to withstand the operating temperature and pressure of the process.
- Elasticity: The material must have the right elastic properties to provide a repeatable and linear response to changes in the displacer’s weight.
9. How does a displacer level transmitter handle interface level measurement?
A displacer level transmitter can be used to measure the interface level between two immiscible liquids with different specific gravities. The displacer is positioned so that it is always submerged in both liquids. As the interface level changes, the buoyant force on the displacer changes due to the different densities of the two liquids. This change in buoyant force is then measured by the transmitter to determine the interface level.
10. How do you calibrate a displacer level transmitter for interface level measurement?
Calibration for interface level measurement is similar to standard level calibration, but the zero and span are set based on the specific gravities of the two liquids.
- Zero (LRV): The displacer is fully submerged in the lighter liquid.
- Span (URV): The displacer is fully submerged in the heavier liquid.
11. Why is a displacer level transmitter (Leveltrol) more suitable than a static head level transmitter on a separator?
In a separator, there are often two immiscible liquids (e.g., oil and water). A displacer level transmitter is more suitable because it can accurately measure the interface level between these two liquids. A static head (differential pressure) transmitter would have difficulty distinguishing the interface level, especially if the densities of the liquids are close or if there are emulsions present.
12. What are the application limits of a displacer level transmitter?
Displacer level transmitters have some limitations, including:
- Limited Measurement Range: They are typically used for relatively small level variations.
- Susceptibility to Buildup: The displacer and torque tube can be affected by coating or solids buildup from the process fluid.
- Sensitivity to Density Changes: Changes in the specific gravity of the process liquid will affect the accuracy of the measurement.
- Moving Parts: The mechanical components can be subject to wear and tear over time.
13. How does temperature affect the measurement of a displacer level transmitter?
Temperature can affect the measurement in two ways:
- Liquid Density: Changes in temperature will cause the density of the process liquid to change, which in turn will alter the buoyant force on the displacer and affect the level reading.
- Torque Tube: Extreme temperatures can affect the elasticity and performance of the torque tube.
14. How does pressure affect the measurement of a displacer level transmitter?
For liquid level measurement, pressure has a negligible effect on the accuracy of a displacer level transmitter because the buoyant force is primarily dependent on the liquid density, which is not significantly affected by pressure.
15. What are the advantages of using a displacer level transmitter?
- High Accuracy and Reliability: They provide accurate and repeatable measurements.
- Suitable for Interface Level: They are well-suited for measuring the interface between two liquids.
- Wide Range of Applications: They can be used in a variety of liquids, including those that are corrosive or at high temperatures and pressures.
- Not Affected by Foam: The measurement is not significantly affected by the presence of foam on the liquid surface.
16. What are the disadvantages of using a displacer level transmitter?
- Moving Parts: They have mechanical parts that can wear out.
- Limited Span: The measurement range is relatively small.
- Affected by Density Changes: Changes in liquid density can cause errors in the measurement.
- Installation Complexity: They can be more complex to install than some other level measurement technologies.
17. Describe the construction of a displacer level transmitter.
A displacer level transmitter typically consists of the following components:
- Displacer: The sensing element that is suspended in the liquid.
- Torque Tube Assembly: Connects the displacer to the transmitter and converts the change in weight to a rotational motion.
- Transmitter Head: Contains the electronic components that convert the mechanical motion into an electrical signal.
- Cage or Stilling Well (optional): A housing for the displacer to protect it from turbulence.
18. What is the relationship between the displacer’s weight and the liquid level?
The apparent weight of the displacer is inversely proportional to the liquid level. As the liquid level rises and submerges more of the displacer, the buoyant force increases, causing the apparent weight of the displacer to decrease.
19. How is the vertical movement of the displacer converted into a readable output?
The change in the apparent weight of the displacer causes a slight twist in the torque tube. This small rotational movement is detected by the transmitter’s sensor (e.g., a Hall effect sensor or a differential capacitor) and converted into a standard 4-20 mA or other electrical output signal that is proportional to the level.
20. What is a “cage” in the context of a displacer level transmitter?
A cage is an external chamber that is mounted to the side of a vessel. The displacer is installed inside the cage, and the cage is connected to the vessel at the top and bottom. The liquid from the vessel flows into the cage, allowing the displacer to measure the level. The cage serves the same purpose as a stilling well, providing a calm liquid surface for stable measurement and allowing for easy isolation of the transmitter for maintenance.
21. What is the effect of foam on a displacer level transmitter?
Displacer level transmitters are generally not affected by foam because the displacer is submerged in the liquid and the measurement is based on buoyancy, not the surface of the liquid. As long as the foam has a significantly lower density than the process liquid, it will have a negligible effect on the measurement.
22. Can a displacer level transmitter be used for open and closed tank applications?
Yes, displacer level transmitters can be used in both open (atmospheric) and closed (pressurized) tanks. The measurement principle is not affected by the pressure in the tank.
23. What is the typical output signal of a displacer level transmitter?
The most common output signal is a 4-20 mA analog current loop. However, some modern displacer transmitters may also offer digital outputs based on protocols like HART, FOUNDATION Fieldbus, or Profibus.
24. What are the common problems encountered with displacer level transmitters?
Common problems include:
- Buildup on the Displacer: Solids or coating on the displacer can alter its weight and volume, leading to inaccurate readings.
- Incorrect Calibration: Improper zero and span adjustments will result in measurement errors.
- Torque Tube Failure: The torque tube can become fatigued or corroded over time.
- Density Variations: Changes in the process liquid’s density will affect the accuracy of the measurement.
25. How do you troubleshoot a displacer level transmitter?
Troubleshooting steps include:
- Check for Physical Obstructions: Ensure that the displacer can move freely and is not obstructed.
- Inspect for Buildup: Clean any material buildup from the displacer and inside the cage.
- Verify Calibration: Perform a calibration check to ensure the zero and span are set correctly.
- Check for Leaks: Inspect the torque tube and seals for any leaks.
- Verify Liquid Density: Ensure that the specific gravity of the process liquid has not changed significantly.
- Check the Electronics: Verify the power supply and output signal wiring.